Four port circulator having mutually coupled resonant cavities



Aug. 30, 1966 H. SEIDEL 3,270,293

FOUR PORT CIRCULATOR HAVING MUTUALLY COUPLED RESQNANT CAVITIES FiledOct. 31, 1963 2 Sheets-Sheet 1 d GVPOMAGA/Er/C ELizMENl F/G. /A bINVENTOR H. 5.5 IDE L A TTORNE V Aug. 30, 1966 H. SEIDEL 3,270,298

FOUR PORT CIBCULATOR HAVING MUTUALLY COUPLED RESONANT GAVITIES FiledOct. 31, 1963 2 Sheets-Sheet 3 G YROMAGNET/C ELg/ZENT afc aVROMAGNU/cELMEN7'\ United States Patent 3,270,298 FOUR PORT CIRCULATOR HAVINGMUTUALLY COUPLED RESQNANT CAVITIES Harold Seidel, Fanwootl, N.J.,assignor to Bell Telephone Laboratories, Incorporated, New York, N.Y., acorporation of New York Filed Oct. 31, 1963, Ser. No. 320,292 7 Claims.(Cl. 3331.1)

This invention relates to electromagnetic wave transmission systems and,in particular, to four port circulators for use in such systems.

The use of materials having gyromagnetic properties to obtain bothreciprocal and nonreciprocal transmission effects in electromagneticcircuits is widely known and has found numerous applications inpropagation structures of both the waveguide and the two-conductortransmission line types.

Included among these new transmission components that have beendeveloped is the so-called circulator. While structurally circulatorsmay differ substantially from each other, they all have in common theelectrical property of transmitting energy into and out of the variousbranches of the network in rotation. Thus, energy applied to a firstbranch of the circulator is coupled selectively to a second branch,whereas energy applied to the second branch is coupled selectively to athird branch. This process is continued until finally energy applied toa fourth branch is coupled selectively to the first branch, thuscompleting the rotational process.

In United States Patent 2,887,664, issued to C. L. Hogan on May 19,1959, circulator action is obtained by utilizing the nonreciprocalpolarization rotational effects produced by a longitudinally biasedgyromagnetic material. In United States Patent 2,849,685, issued to M.T. Weiss on August 26, 1958, the nonreciprocal phase shift produced by atransversely biased gyromagnet-ic material is utilized to producecirculator action.

In accordance with the present invention, large nonreciprocaldissipation is utilized to produce low-loss nonreciprocal transmissioneffects and, in turn, to produce a four port circulator. Moreparticularly, a resonantly biased gyromagnetic element is coupled to apair of suitably oriented resonant cavities. Transmission through thecavities occurs in that direction of propagation consistent with lowdissipation. For propagation in the opposite direction, the large lossycomponent introduced into the cavities by the resonantly biasedgyromagnetic element results in a destruction of the resonant propertiesof the cavities and a virtually complete reflection of the incident waveenergy.

In an illustrative embodiment of the invention utilizing the principlesdescribed above, two 3 db quadrature hybrids are employed. One pair ofconjugate branches of one of the hybrids is connected to a pair ofconjugate branches of the other hybrid by means of a pair oforthogonally oriented resonant cavities. The cavities are mutuallycoupled to a magnetically biased element of gyromagnetic material whosebiasing field is adjusted to produce gyromagnetic resonance at theresonant frequency of the two cavities. Because of the 90 degree phasedelay introduced by the quadrature hybrids and the orthogonal spatialorientation of the cavities, a region of circularly polarized magneticfield is established at the gyromagnetic element. The sense of rotationof the magnetic field is a function of the direction of propagation ofthe incident wave energy. For one direction of propagation, and onesense of rotation, energy is freely propagated through the resonantcavities, whereas for propagation in the opposite direction the sense ofrotation is reversed, and the incident energy is reflected. The energycomponents Patented August 30, 1966 ice so transmitted or reflected arerecombined in the hybrids to produce circulator action.

The invention as described above can be embodied using hollow,condu-ctively bounded waveguides, or twoconductor transmission lines, aswill be described in greater detail hereinafter.

These and other objects and advantages, the nature of the presentinvention, and its various features, will appear more fully uponconsideration of the various illustrative embodiments now to bedescribed in detail in connection with the accompanying drawings, inwhich:

FIG. 1 is a block diagram of a circulator in accordance with theinvention;

FIG. 1A shows the sequence of coupling between branches of thecirculator of FIG. 1;

FIG. 2, given for purposes of explanation, shows the signal magneticfield components and the biasing field in the region of the gyromagneticmaterial;

FIG. 3 is an illustrative embodiment of the invention using rectangularwaveguides; and

FIG. 4 is an illustrative embodiment of the invention using striptransmission lines.

Referring to FIG. 1, there is illustrated, in block diagram, acirculator in accordance with the invention. The circulator comprises apair of 3 db quadrature hybrids 10 and 11, each of which has two pairsof conjugate branches. The pairs of conjugate branches associated withhybrid 10 are designated ab and cd. The pairs of conjugate branchesassociated with hybrid 111 are designated a-b' and cd.

The term 3 db quadrature hybrid refers to that class of power dividingnetworks in which the power of the incident signal applied to one branchof one pair of conjugate branches divides equally between the other pairof conjugate branches and wherein the relative phases of the dividedsignals differ by degrees. This includes a large variety of powerdividing networks among which are the Riblet coupler (H. J. Riblet, TheShort-Slot Hybrid Junction, Proceedings of the Institute of RadioEngineers, vol. 40, No. 2, February 1952, pages to 184), the multiholedirectional coupler (8. E. Miller, Coupled Wave Theory and WaveguideApplications," Bell System Technical Journal, vol. 33, May 1954, pages661 to 719), the semi-optical directional coupler (E. A. J. Marcatili, ACircular Electric Hybrid Junction and Some Channel- Dropping Filters,Bell System Technical Journal, vol. 40, January 1961, pages 185 to 196),and the strip transmission line directional coupler (I. K. Shimizu,Strip-line 3 db Directional Couplers, published in the 1957 Institute ofRadio Engineers Wescon Convention Record, vol. 1, Part 1, pages 4 to15).

In each of the above-mentioned power dividing networks there is a 90degree relative phase shift between the output wave components. This isindicated by the 40 and 490 designations between adjacent branches ofeach of the hybrids 10 and 11.

To avoid unnecessary repetition, the terms directional coupler or hybridwhen used hereinafter shall be understood to mean a 3 db quadraturehybrid having the characteristics referred to above.

Referring again to FIG. 1, branch 0 of hylbrid '10 is connected tobranch 0' of hydrid 11 by means of a first resonant member =12.Similarly, branch at of hybrid 10 is connected to branch d of hybrid 1:1by means of a second resonant member 13. Members 12 and 13, which arepreferably identical, are spacially oriented with respect to each otherto produce a region of orthogonally directed magnetic fields. Amagnetically biased element of gyromagnetic material 14 is locatedwithin that region. Typically, resonant members 12 and 13 aretransmission line cavities and are oriented at right angles to eachother.

The term gyromagnetic material is employed here in its accepted sense asdesignating the class of magnetically polarizable materials havingunpaired spin systems involving portions of the atoms thereof that arecapable of being aligned by an external magnetic polarizing field andwhich exhibit a precessional motion at a frequency with the rangecontemplated by the invention under the combined influence of saidpolarizing field and an orthogonally directed varying magnetic fieldcomponent. This precessional motion is characterized as having anangular momentum, and a magnetic moment. Typical of such materials areionized gases, paramagnetic material and ferromagnetic materials, thelatter including the spinels such as magnesium aluminum ferrite,aluminum zinc ferrite and the rare earth iron oxides having agarnet-like structure of the formula A B O where O is oxygen, A is atleast one element selected from the group consisting of yttrium and therare earths having an atomic number between 62 and 71, inclusive, and Bis iron optionally containing at least one element selected from thegroup consisting of gallium, aluminum, scandium, indium and chromium.

Both circuits 12 and 13 are tuned to resonance at the operatingfrequency. If a band of frequencies are involved, the circuits are tunedto resonance at the midband frequency. In addition, the gyrom'agneticelement is biased to gyromagnetic resonance at that same frequency bymagnetic means not shown.

The operation of the circulator shown in FIG. 1 is based upon thenonreciprooal behavior of gyromagnetic materials. As is well known,magnetically polarized gyromagnetic materials exhibit distinctlydifferent properties depending upon the nature of the applied magneticfields. These properties are the result of the fact that the magneticmoment of the material tends to precess in a pretferred sense andresists rotation in the opposite sense. As a consequence, circularlypolarized magnetic fields influence the gyromagnetic materialdifferently depending upon their sense of rotation. This is so since acircularly polarized magnetic field rotating in one direction isrotating in the easy angular direction of precession of the magneticmoment, whereas an oppositely rotating circularly polarized magneticfield is rotating in a sense inconsistent with the natural behavior ofthe magnetic moment of the gyromagnetic material. Therefore, when theeflfective magnetic field of the signal wave rotated in the same senseas the preferred direction of precession of the magnetic moment, itcouples strongly to the gyromagnetic material. However, very littlecoupling takes place between the external magnetic field and thegyromagnetic material when the effective signal magnetic field isrotating in the opposite angular direction.

Referring again to FIG. 1, the gyromagnetic element 14 is coupled tocavity 12 in a region of the cavity having a substantial magnetic fieldcomponent. It is also coupled to a corresponding region of cavity 13having an equal magnetic field intensity. Since the cavities areoriented at right angles to each other, the magnetic field componentsproduced by the two cavities are similarly orthogonally oriented.

In addition to the spacial orthogonality of the two fields, there is atime orthogonality produced by the polwer dividing networks and 11. Thecombined effect of these two orthogonalities is to produce theequivalent of a circularly rotating magnetic field at the gyromagneticmaterial.

FIG. 2, given for purposes of explanation, shows the orientation andphase relationships of the magnetic field components at the gyromagneticmaterial.

Assuming that signal is applied to branch a of hybrid 10, half of theincident power is coupled to branch c and the remaining half to branchd. In addition, there is a 90 degree time phase ldiiferen'ce between thetwo equal wave components. These waves excite cavities 12 and 13 ninetydegrees out of time phase with respect to each other. In addition, themagnetic field components are in space quadrature. This is illustratedin FIG. 2, which S'l'lOlWS a first field component from cavity 12indicated by arrow 12', and a second field component from cavity L3indicated by arrow 13. Because of the orientation of the cavities thesecomponents are perpendicular to each other. Also shown is the biasingfield component H which is applied in a direction perpendicular to theplane defined by components 12' and 13 Because the cavities are exciteddegrees out of time phase, component 18' is a minimum when component 12'is a maximum. As component 13 increases in amplitude, component 12decreases. As illustrated in FIG. 2, component 13' is pictured asincreasing in the indicated direction while component 12 is decreasing.The effect of field components 12' and 13 varying in this manner is toproduce the equivalent of a single, resultant field component whichappears to rotate in space in the region occupied by the gyromagneti-celement 14. With the biasing field H directed as shown, a negative, orcounterclockwise rotation is produced when viewed along the direction ofthe biasing field. As this sense of rotation is opposite to the naturalprecessional sense of the material, little or no interaction takes placebetween the signal wave and the gyromagnetic material. Substantially allof the wave energy applied to cavities '12 and 13 continues to propagatethrough the cavities to hybrid 11, [where the two coimponents recombineand leave the circulator through branch a.

For wave energy applied to branch a of hybrid 11, the relative timephase difference between magnetic field components 12 and 13 is suchthat a positive, or clockwise sense of rotation is produced at thegyromagnetic material. This produces a strong interaction with thegyromagnetic material. Since the latter is biased to gyromagneticresonance, a large lossy component is introduced into each of thecavities. The effect of this lossy component is to detune the cavities,thereby causing a large impedance mismatch at the inputs to the cavitieswhich, in turn, causes substantially all of the incident wave energy tobe reflected back towards hybrid 11. The two reflected wave componentsrecombine and leave by way of branch b.

Similarly, it can be shown that wave energy applied at branch b leavesthe network by way of branch b whereas wave energy applied at branch bleaves the network by way of branch a, thus providing typical circulatoraction. The sequence of coupling between branches is thus aa'b-b, asshown in FIG. 1A.

FIGS. 3 and 4 are specific illustrative embodiments of the inventionbroadly described above. The embodiment of FIG. 3 illustrates theinvention in a system using conductively bounded waveguides, whereas theembodiment of FIG. 4 illustrates the invention in a system using striptransmission lines.

Referring to FIG. 3, a pair of distributed hole directional couplers 30and 31 are used as the 3 db qnadra-' ture hybrids. Each comprises a pairof rectangular waveguides of substantially equal cross-sectionaldimensions aligned parallel to each other and sharing a common narrowwall. More specifically, directional coupler 30 is made up of Waveguidesections 32 and 33 which share a common narrow wall 34. Directionalcoupler 31 is made up of waveguide sections 35 and 36 which share acommon narrow wall 37.

Distributed along the common walls 34 and 37 are the coupling apertures38 and 39, respectively. The size and distribution of the couplingapertures are designed in accordance with procedures well known in theart as. are described in the above-mentioned article by S. E. Miller.

Each of the directional couplers has two pairs of conjugate branchesa-b, cd and a'b', c'-a".

Branch 6 of coupler 30 and branch c of coupler 31 are connected tocavity 40 by means of waveguide sections 42 and 43, respectively.Similarly, branch d of coupler and branch (2' of coupler 31 areconnected to cavity 41 by means of waveguide sections 44 and 45,respectively. Preferably sections 42, 43, 44 and 45 have the sameelectrical lengths.

Cavities and 41 are sections of dominant mode rectangular waveguide,bounded at each end by means of transversely extending conductivemembers 50, 51, 52 and 53 each of which has a coupling aperture forcoupling wave energy into and out of the respective cavities.

Cavities 40 and 41 cross each other such that a portion of the lowerwide wall of cavity 40 is contiguous with a corresponding portion of theupper wide wall of cavity 41. In particular, the two cavities cross eachother at right angles. That is, the longitudinal axis of cavity 40 isperpendicular to the longitudinal axis of cavity 41.

Extending through the contiguous wide walls of cavities 40 and 41 is anaperture 54 in which there is located an element of gyromagneticmaterial 55. The element 55 is shown as a sphere. However, it may assumeany other convenient shape since its particular physical configurationis not critical to the operation of the invention.

A static magnetic biasing H is applied perpendicular to the wide wallsof the cavities and is adjusted to produce gyromagnetic resonance inelement 55 at the operating frequency. If the device is to operate overa band of frequencies, gyromagnetic resonance is induced at the mid-bandfrequency.

The biasing field H can be supplied by any suitable means (not shown),such as an electric solenoid, a permanent magnet, or the gyromagneticmaterial itself can be permanently magnetized.

Advantageously, the gyromagnetic material is located in a region of highmagnetic field intensity. In the embodiment of FIG. 3, the cavities aremade to have an electrical length of one wavelength, and thegyromagnetic material is located in the center of the cavities. In thisposition the gyromagnetic element is in a region of maximum transversemagnetic field intensity. Alternatively, the gyromagnetic material canbe coupled to longitudinal magnetic field components.

As indicated above, the principles of the invention can be applied toother types of transmission media. Another application is illustrated inthe embodiment of FIG. 4 which utilizes strip transmission lines. Inthis embodiment, 2. pair of directional couplers 60 and 61, of the typedescribed in the above-mentioned article by J. K. Shimizu, are used asthe quadrature power divided networks.

As before a pair of conjugate branches of each directional couplers c-dand c-d are connected to a pair of orthogonally oriented resonantcavities 62 and 63. Each cavity comprises a length of conductivelyinsulated line Whose electrical length is a multiple of half awavelength at the operating frequency. An element of gyromagneticmaterial 64 is located between the cavities and means for magneticallybiasing the element are provided. The mode of operation of thisembodiment of the invention is the same as described hereinabove.

In all cases it is understood that the above-described arrangements aresimply illustrative of but a small number of the many possible specificembodiments which can represent applications of the invention. Forexample, other types of 3 db quadrature hybrids and other cavityconfigurations can be used. In addition, two or more sets of cavitiescan be used to increase the isolation between noncoupled branches. Thus,numerous and other varied arrangements can readily be devised inaccordance with these principles by those skilled in the art Withoutdeparting from the spirit and scope of the invention.

What is claimed is:

1. A four port circulator comprising:

two 3 db quadrature hybrids each having two pairs of conjugate branches;

a pair of transmission line resonant members supportive ofelectromagnetic wave energy connecting one pair of conjugate branches ofone of said bybrids to one pair of conjugate branches of the other ofsaid hybrids;

said resonant members being physically oriented with respect to eachother to produce when energized a region of orthogonally intersectingmagnetic fields;

and an element of magnetically biased gyromagnetic material locatedwithin said region.

2. The circulator according to claim 1 wherein said hybrids and saidresonant members comprise sections of strip transmission line.

3. A four port circulator comprising:

two 3 db quadrature hybrids each having two pairs of conjugate branches;

a pair of transmission line resonant cavitiestuned to the same resonantfrequency connecting one pair of conjugate branches of one of saidhybrids to one pair of conjugate branches of the other of said hybrids;

said cavities crossing each other at right angles at a region alongtheir respective lengths;

and an element of gyromagnetic material magnetically biased togyromagnetic resonance at said same frequency electromagneticallycoupled to said crossed cavities.

4. The circulator according to claim 3 wherein said hybrids and saidcavities comprise sections of conductively bounded waveguide.

5. The circulator according to claim 4 wherein each of said cavitiescomprises a length of conductively terminated waveguide;

and means for coupling into and out of each end of said cavities.

6. The circulator according to claim 5 wherein said element ofgyromagnetic material is coupled to corresponding regions of saidcavities at an electrical distance from said terminations equal to anintegral multiple of half a wavelength at said resonant frequency.

7. In an electromagnetic wave transmission system, a four portcirculator comprising:

two 3 db directional couplers each having two pairs of conjugatebranches;

means for connecting the branches of one pair of conjugate branches ofone directional coupler to the branches of one pair of conjugatebranches of the other of said directional couplers comprising a pair oftuned transmission line members;

said members being supportive of electromagnetic wave energy havingtransversely directed magnetic field components;

said members being oriented with respect to each other with thetransversely directed. components of each being orthogonal to thetransversely directed components of the other;

an element of gyromagnetic material positioned with respect to saidmembers so as to couple to a region of said orthogonally directed fieldcomponents;

and means for magnetically biasing said element to gyromagneticresonance at the frequency of said members.

References Cited by the Examiner UNITED STATES PATENTS 2,951,216 8/1960Nelson et al. 3331.1

3,162,826 12/1964 Englebrecht 333l.l

HERMAN KARL SAALBACH, Primary Examiner.

P. L. GENSLER, Assistant Examiner.

1. A FOUR PORT CIRCULATOR COMPRISING: TWO 3 DB QUADRATURE HYBRIDS EACHHAVING TWO PAIRS OF CONJUGATE BRANCHES; A PAIR OF TRANSMISSION LINERESONANT MEMBERS SUPPORTIVE OF ELECTROMAGNETIC WAVE ENERGY CONNECTINGONE PAIR OF CONJUGATE BRANCHES OF ONE OF SAID HYBRIDS TO ONE PAIR OFCONJUGATE BRANCHES OF THE OTHER OF SAID HYBRIDS; SAID RESONANT MEMBERSBEING PHYSICALLY ORIENTED WITH RESPECT TO EACH OTHER TO PRODUCE WHENENERGIZED A REGION OF ORTHOGONALLY INTERSECTING MAGNETIC FIELDS; AND ANELEMENT OF MAGNETICALLY BIASED GYROMAGNETIC MATERIAL LOCATED WITHIN SAIDREGION.